A practical application of an assembly as mentioned is in an engine-driven ship which is equipped with a box cooler for cooling the fluid of an engine cooling system of the ship, the box cooler comprising a plurality of tubes for containing and transporting the fluid to be cooled in their interior. Typically, such a ship has a compartment for accommodating the tubes of the box cooler, wherein the compartment is defined by a portion of the hull of the ship and partition plates, and wherein entry and exit openings are arranged in the hull at the position of the compartment so that water can enter the compartment, flow over the tubes in the compartment, and exit the compartment through natural flow and/or under the influence of motion of the ship.
A box cooler is a specific type of heat exchanger which is designed for use in an engine-driven ship. For example, in the case of a tugboat having an installed engine power of 15 MW, one or more box coolers are applied for transferring heat in the order of 5 MW to the seawater. Usually, a box cooler comprises bundles of U-shaped tubes for conducting a fluid to be cooled, wherein ends of leg portions of the tubes are secured to a common plate having openings for providing access to both leg portions of each of the tubes. It is a very practical option to enable the box cooler to perform its cooling function by continuously exposing the tubes thereof to fresh seawater. However, the environment of a box cooler is ideally suited for a phenomenon known as biological fouling or biofouling, as the seawater is heated to a medium temperature in the vicinity of the tubes as a result of the heat exchange with the relatively hot fluid in the interior of the tubes, and the constant flow of water continuously brings in new nutrients and organisms which are known to cause biofouling.
In general, biofouling is the accumulation of microorganisms, plants, algae, small animals and the like on surfaces. According to some estimates, over 1,800 species comprising over 4,000 organisms are responsible for biofouling. Hence, biofouling is caused by a wide variety of organisms, and involves much more than an attachment of barnacles and seaweeds to surfaces. Biofouling is divided into micro fouling which includes biofilm formation and bacterial adhesion, and macro fouling which includes the attachment of larger organisms. Due to the distinct chemistry and biology that determine what prevents them from settling, organisms are also classified as being hard or soft. Hard fouling organisms include calcareous organisms such as barnacles, encrusting bryozoans, mollusks, polychaetes and other tube worms, and zebra mussels. Soft fouling organisms include non-calcareous organisms such as seaweed, hydroids, algae and biofilm “slime”. Together, these organisms form a fouling community.
In several situations, bio fouling creates substantial problems. Bio fouling can cause machinery to stop working, water inlets to get clogged, and heat exchangers to suffer from reduced performance. Hence, the topic of anti-fouling, i.e. the process of removing or preventing bio fouling, is well-known. In industrial processes involving wetted surfaces, bio dispersants can be used to control biofouling. In less controlled environments, fouling organisms are killed or repelled with coatings using biocides, thermal treatments or pulses of energy. Nontoxic mechanical strategies that prevent organisms from attaching to a surface include choosing a material or coating for causing the surface to be slippery, or creating nanoscale surface topologies similar to the skin of sharks and dolphins which only offer poor anchor points.
Biofouling of box coolers causes severe problems. The main issue is a reduced heat transferring capability as layers of bio fouling are effective heat insulators. When the bio fouling layers are so thick that seawater can no longer circulate between adjacent tubes of the box cooler, an additional deteriorating effect on the heat transfer is obtained. Thus, biofouling of box coolers increases the risk of engine over-heating, so that ships need to slow down or ship engines get damaged.
Anti-fouling arrangements for cooling units that cool the water from a cooling water system of an engine-driven ship by means of seawater are known in the art. For example, DE 102008029464 relates to a box cooler for use in ships and on offshore platforms, comprising an integrated anti-fouling system for killing fouling organisms by means of an overheating process that can be regularly repeated. In particular, the box cooler is protected against microorganism fouling by continuously overheating a defined number of heat exchanger tubes without interrupting the cooling process, wherein waste heat from the cooling water may be used for doing so.
In general, it is known in the art to use ultraviolet light for removing/preventing the formation of biofilm on wet surfaces. For example, WO 2014/014779 discloses a system for reducing fouling of a surface of an optically transparent element subjected to a marine environment, including a LED for emitting ultraviolet radiation, a mount for directing emitted ultraviolet radiation toward the optically transparent element, and control circuitry for driving the LED.
When it comes to keeping the tubes of a box cooler free from bio fouling, it is possible to have a box cooler which is equipped with a plurality of ultraviolet light sources which are positioned in an effective arrangement with respect to the tubes so as to be capable of casting ultraviolet light over the entire exterior surface of the tubes with an intensity which is sufficient for obtaining the desired effect of keeping the tubes clean. Such a type of box cooler is very well suitable to be used in practice. Nevertheless, a number of problems are associated with the use of the ultraviolet light sources at positions in the box cooler and right outside the box cooler. In the first place, it is a fact that the ultraviolet light sources, or the casings of the ultraviolet sources if a type of energy source comprising an ultraviolet light source and a casing for accommodating the light source is applied, are contacted by water, namely the water which is used for cooling the tubes of the box cooler. It may even be so that the energy sources are submerged in water most of their lifetime. On the basis of this fact, it is necessary to use waterproof electrical cables and to provide for watertight connections. Furthermore, on the basis of this fact, the energy sources are susceptible to fouling themselves, especially fouling caused by mineral deposits. In the second place, the energy sources are very hard to replace, if it is at all possible to do so. The compartment of the ship where the box cooler is present can only be reached by divers from underneath the ship, or during dry docking of the ship. Replacing the energy sources under water is a bothersome job and includes challenges in the field of electrical connections.